1. Field of the Invention
The present invention relates to an imaging apparatus. More particularly, the present invention relates to an imaging apparatus such as a complementary metal oxide semiconductor (CMOS) image sensor which is used in, e.g., a camera-equipped mobile phone, a digital camera, or a video camera.
2. Description of the Related Art
In an imaging apparatus, a random noise suppressing method using a median filtering processing has been conventionally proposed (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 4-235472). In this proposed medial filtering processing, a signal of a central pixel in a 3×3 pixel region is substituted by a median (signal which is a fifth signal counted from the highest or lowest level) obtained by sorting signals in the 3×3 pixel region in order of level magnitude. Therefore, there is a problem that a resolution at an edge portion of an image is degraded or a sufficient random noise suppressing effect cannot be obtained when random noise of an imaging system is large. In order to enable sufficient suppression of the random noise, a pixel region must be increased, and a circuit scale thereby becomes large.
Further, there has been proposed a method of judging a level difference between an average value of signals of pixels around a central pixel and a signal of the central pixel and substituting the signal of the central pixel in accordance with a result of this judgment (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2002-10108). According to this method, when the level difference is smaller than a set threshold value, noise is recognized, and the signal of the central pixel is substituted by the average value of the signals of the peripheral pixels. Conversely, when the level difference is larger than the threshold value, the signal of the central pixel is not substituted (used as it is). However, according to this method, when a defect is produced in one given pixel, noise due to this defect cannot be suppressed. When suppressing the noise due to the defect, another dedicated circuit is required, resulting in a problem of an increase in a circuit scale.
Furthermore, a random noise suppressing method of detecting edge information of an image and performing median filtering processing has been proposed (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2005-303731). However, in case of this proposal, four difference circuits are required to extract an edge. Further, there is also a problem that random noise cannot be suppressed when an edge is determined. Moreover, as with the above-explained proposal, since a signal of a central pixel is substituted by a median, random noise cannot be sufficiently suppressed.